Once-through vapor generator with division wall



United States Patent [72] Inventors JosephG.'Altrnan Dover; JanLJrledrichJomptonPlalns, NJ. [2l] Appl.No. 793,303 [22] Filed Jan. 23, 1969 3,344,777 10/1967 Gorzegno 122/406 Primary Examiner- Kenneth W. Sprague Attorneys-John Maier Ill and Marvin A. Naigur ABSTRACT: A supercritical forced-flow once-through vapor generator comprising a radiantly heated rectangular fumace enclosure which includes a plurality of parallel vertically oriented tubes defining a first fluid pass of front and rear wall panels of the enclosure, and in series therewith a second fluid pass of sidewall panels of the enclosure. The enclosure has a lower high-absorption burner zone and an upper gas exit, the generator including a convection area in gas flow communication with said upper gas exit. A partial division wall including at least one panel of tubes is positioned adjacent said upper gas exit, the generator including header connection means connecting said partial division wall so that it is upstream of said first fluid pass. The furnace enclosure is sized to limit the enthalpy pickup therein by utilizing the maximum permissible flue gas exit temperature, for the fuel fired, the furnace circuits affording a practical minimum pass furnace design which is applicable where the furnace enclosure fluid enthalpy pickup is considerable.

PATENTED UEC22I97U 3548788 sum 1 0F 7 INVENTORS. JOSEPH CALTMAN 8*:

NHQN L. FFREDRECH PAIENIED M022 I970 SHEET 2 BF 7 FIG ECONOPEHZ E R TO FURNACE ENCLOSURE SECOND PASQ INVENTORJS. JO'EPH C. ALTMAN a L. FREEGF-EKCH TO FURNACE ENCLOSURE FIRST PASS NO MIXHEADER PATENTEU DEE? 219m FIG.

JOSEPH CBALTIVIAN Es JQN FREEDFZEQH MIX HEADERS FIG. 9

50 3 mm as? JOSEPH G-ALTMAN 81 JAN Ln FRIEDRiCH ONCE-THROUGH VAPOR GENERATOR WITH DIVISION WALL DESCRIPTION The present invention relates to vapor generators, and in particular to vapor generators of the once-through type.

The invention is particularly applicable to the Benson" type once-through vapor generator design, and will be described with reference thereto, although .it will be appreciated that the invention has broader application, such as with the Sulzer? design or with the recirculation-type of generator.

The invention also is particularly applicable to a supercritical once-through vapor generator.

A once-through vapor generator of the Benson" design is one wherein the fluid flow is forced through tubes of the generator without recirculation. The basiccircuitry of this Benson" design consists of heated upflow tubes coupled to unheated downcomers. ln once-through vapor generators of past years, the fluid flow was transmitted frequently in at least a single first-flow pass defining the entire perimeter of the generator furnace enclosure walls, to passes making up the remainder of the furnace and convection enclosure circuitry of the generator, the convention enclosure, area including superheating passes; and from there to a point of use, all of the passes being connected in series with each other. The convection area usually extended from the top of the furnace enclosure, and burners for the generator were disposed near the bottom of either or both of the front and rear walls of the enclosure.

Once-through vapor generators are becoming larger in capacity and dimension, present generators having a furnace enclosure of very large dimension, with a large number of burners disposed in opposite front and rear walls of the enclosure. Because of this large size, special precautions have to be taken to insure equal heat input distribution and a correspondingly equal distribution of fluid flow in tubes of the furnace enclosure. For instance, tubes in the center-of a furnace enclosure wall may experience more heat absorption than tubes in the comer of the enclosure, resulting in unequal temperatures in the periphery of the enclosure and an imbalance in the flow. This imbalance, could result in a relatively stagnant flow in part of the enclosure, in turn quickly resulting in tube overheating.

In general, parallel tube forced-flow circuit where fluid enthalpy pickup is large exhibit greater sensitivity to flow imbalance caused by heat absorption upset. A furnace pass (fluid circuit) encompassing the entire furnace periphery is more subject to flow imbalance, because the enthalpy pickup is usually correspondingly greater, and the geometry of the pass arrangement imposes greater absorption heat upsets.

A member of different approaches have been taken in the past to reduce or overcome the problem of sensitivity of a furnacecircuit to heat absorption upset. One approach used has been to recirculate part of the fluid flow leaving the furnace back to the furnace inlet end. The furnace circuit when this approach has been used has consisted of a single flow pass encompassing the entire furnace periphery. An increase in the fluid weight flow through the pass by recirculation maintains fluid velocities at lower loads and reduces fluid enthalpy pickup in direct relation to the quantity recirculated. This approach has the disadvantage in that it increases pumping costs and power losses to pump the recirculated fluid, and also adds to maintenance requirements.

in addition, recirculation is usually not economical at full load operating conditions, so that the furnace at full load operates with substantial fluid enthalpy pickup plus the necessity to distribute fluid flow and heat absorption to the full furnace periphery.

It has also been proposed to limit the enthalpy pickup in the furnace enclosure, and thereby reduce the danger of a flow imbalance caused by differences in tube enthalpy pickups, by recirculating flue gases to the enclosure. The cooler flue gasses mix with the burner gases lowering the gas temperatures in the furnace, in turn lowering furnace absorption, and the resulting fluid enthalpy pickup. Gas recirculation however has the obvious disadvantage in that it increases equipment, construction, operating and maintenance costs.

A further proposal has been to divide the furnace enclosure into a plurality of parallel oriented upflowheated passes, each comprised of parallel tubes, with means to connect the passes in series and distribute the flow uniformly to the tubes of the passes. Each pass has a fewer number of parallel tubes, and correspondingly less fluid enthalpy pickup reducing the likelihood of a flow upset in any one pass. This design also makes possible higher fluid mass flow rates within the tubes, without fluid recirculation, yielding a more conservative lower tube metal temperature. These parallel oriented upflow heated furnace passes connected in series, with unheated downcomers between the passes, adhere to the concept of the Benson principle of design.

inherently the above arrangement of many furnace passes in series results in increased capital costs in manufacture and construction for the generator because of the size and number of downcomers, headers, and connection pipes. For instance, the downcomers and connection pipes associated therewith required between the multiple flow passes in series, add to the length of the fluid flow path. Because of this added flow path length, the downcomers and connection pipes, and in some cases headers, must be sized sufficiently large to limit the fluid pressure drop to within acceptable limits.-

A factor which also complicates the use of the multipass design is the all-welded wall construction in which parallel finned tubes are welded together along their lengths to provide a gastight enclosure. Although this construction has resulted in substantial savings in construction costs, eliminating the use of complex casing designs, it has meant that the passes must be arranged so that adjacent tubes are at roughly the same temperature to avoid fracture of the connections between the tubes, or of the tubes themselves, caused by thermal stresses resulting from the restrained growth of one tube relative to another during load changes in the generator.

As a rule of thumb, a F. to l25 F. maximum tube temperature difference is allowed between adjacent tubes of an all-welded generator furnace enclosure.

in an application, Ser. No. 794,629, flied Jan. 28, 1969, assigned to assignees of the present application, which is copending with the present application, entitled Two Pass Furnace Circuit Arrangement for Once-Through Vapor Generator, in the names of Walter P. Gorzegno, William D. Stevens and Jan L. Friedrich, an improved vapor generator design is set forth in which the conservativeness of the multipass design is substantially met, and which at the same time substantially matches the cost advantage of the type of furnace enclosure having only a single upflow pass. This copending application proposes a minimum pass furnace circuit design which affords a practical furnace design approach from the standpoint of functional adequacy for the fuel and furnace design criteria imposed. The invention relates to a furnace circuit design where the fuel fired gives a maximum permissible furnace exit gas temperature of a relatively high value, resulting in minimum furnace absorption. For these conditions, a two pass arrangement is sufficient to limit enthalpy pickup per pass to a value which provides stable circuit characteristics for the supercritical fluid; the enthalpy pickup being sufficiently low to reduce the likelihood of a flow imbalance caused by a heat upset; the geometry also provides mass flow rates sufficiently high to maintain adequate cooling or conservative tube metal temperatures of the furnace circuit tubes should a flow imbalance occur.

in addition, this furnace design wherein the flue gas temperature at the furnace enclosure exit is the maximum permissible for the fuel fired, has the affect of in turn inc easing the log-mean temperature difference between the heating flue gases and superheater and reheater sections in the convection area of the generator. This increases the efliciency of heat transfer for these surfaces and has the overall affect of increasing the rating of the generator permitting reduction in the generator overall size per kilowatt hour produced.

By having a furnace enclosure of just two passes, costs are substantially reduced through the elimination of downcomers, headers, and connections, and in the greater simplicity of construction.

It was stated in the eopending application that the invention was primarily for gas and oil firing; which allow high heat releases permitting the design of a small sized highly rated furnace enclosure.

The present invention adopts essentially the same inventive principles, of the eopending application, to firing of coal, wherein the permitted heat release is less because of the nature of the fuel requiring more furnace heat transfer surface or area.

In accordance with the invention, there is provided a supercritical forced-flow once-through vapor generator comprising a radiantly heated rectangular furnace enclosure which includes a plurality of parallel vertically oriented tubes defining front, rear and sidewall panels, header means connecting said panels in series into first and second flow passes, the first flow pass comprising at least part of the enclosure front and rear wall panels, the second flow pass comprising at least part of the enclosure sidewall panels. The enclosure includes a lower high absorption burner zone and an upper gas exit, the generator further including a convention area in flow communication with said gas exit, and a water cooled partial division wall in eluding at least one panel of tubes positioned in the furnace enclosure adjacent the gas exit, the division wall including header connection means connecting said division wall panel upstream of the furnace enclosure first pass.

The furnace enclosure is sized to limit the enthalpy pickup therein so that the maximum permissible flue gas exit temperature, for the fuelfired, is obtained. By maximum permissible flue gas exit temperature it is meant the upper design limit for the flue gas temperature at the furnace gas exit. The particular design value for this limit varies depending primarily upon the fuel fired, but for a particular fuel, and defined conditions, the limit generally is fixed and well known. In the case of a gas fired unit, the accepted limit is about 2,800 F above which the residence time in the furnace may be so short as to preclude complete combustion in the furnace; and above which the cost for alloy tubes in for instance a superheater circuit adjacent the gas exit becomes economically unattractive. In the case of oil firing, vanadium attack on metals in the convention zone sets the limit at about 2,650 F. For coal, the limit is even lower, less than about 2,350 F., depending upon the grade of coal, above which slagging at the flue gas exit can occur. It is of course understood that the above values may vary somewhat depending upon a number of factors, and that these values are only representative.

The principle factor which enters into obtaining a maximum permissible flue gas exit temperature" is sizing of the generator furnace so that the furnace enthalpy pickup or heat absorption in the furnace is correspondingly small. However, the relatively low maximum permissible flue gas exit temperature, or relatively low gas exit temperature limit which is set for coal firing, as compared to that for gas or oil firing, requires that the furnace enthalpy pickup for coal firing be correspondingly greater; and that the furnace be sized with a correspondingly greater amount of absorption surface area.

The present invention provides a practical furnace circuit and enclosure minimum pass approach which is applicable where the furnace fluid enthalpy pickup is considerable.

In particular, the use of division wall panels, connected into the furnace circuitry upstream of the furnace enclosure first and second passes, comprising an amount of surface area sufficient to absorb a substantial amount of heat, lowers the enthalpy pickup required of the furnace enclosure passes to the extent that the two passes alone can make up the furnace enclosure, without the enthalpy pickup of either pass being in excess of that required from a flow stability point of view.

Each pass has an enthalpy pickup which is sufiiciently low so that any flow imbalance in the pass caused by a heat upset is correspondingly small.

In addition, by so limiting the size of the furnace enclosure, the furnace enclosure geometry, defined as distribution of flow passes and selection of tube sizes, can be set so as to obtain the high mass flow rates necessary to render the furnace circuitry relatively insensitive to flow imbalance; that is,'sufficient mass flow to obtain proper cooling of the tubes even when a heat absorption upset and resulting flow imbalance occur.

As with the eopending applicationSer. No. 794,629, by limiting the supercritical fluid enthalpy pickup in the furnace, and having a correspondingly high gas exit temperature, the log-mean temperature difference between the heating flue gases and supercritical fluid in the convection super heater and reheater is increases to maximum values, increasing the efficiency of heat transfer for these expensive sections, permitting them to be reduced in size. This has the effect of increasing the generator overall rating (defined as B.t.u. per hr.- sq. ft. of surface area).

Preferably, the convection enclosure portion of the generav tor comprises a horizontally extending vestibule, and a downwardly extending convection area to define an inverted L-shaped configuration, the generator further including platen and finishing superheating surfaces, the platen superheating surface being positioned in the furnace enclosure adjacent the gas exit outlet, the finishing superheating surface being positioned in the vestibule.

It is further preferred that the partial division wall be comprised of a plurality of L-shaped panels arranged in side-byside spaced relationship between the sidewalls of the furnace enclosure including substantially horizontally extending legs or portions penetrating the rear wall of the enclosure and vertically extending legs or portions penetrating the roof of the enclosure.

Alternately, these partial division wall panels may consist of substantially horizontally extending legs or portions penetrating the front wall of the enclosure and vertically extending legs or portions penetrating the roof of the enclosure.

An advantage of this type of division wall is that it offers more surface area than a full length single panel division wall, further permitting a reduction in the size of the furnace enclosure in accordance with the present invention. In addition the partial division walls afford a means for obtaining sufl'icient flow area for a large unit by simply increasing the number of partial division wall panels used.

If desired in accordance with the invention, the first and second flow passes may be arranged to occupy only the lower portions of the front, rear and sidewalls of the furnace enclosure, the tubes of the furnace enclosure being divided into a third flow pass connected in series with the second flow pass, occupying the upper furnace front and sidewalls. Downcomer and connection means connect the furnace enclosure roof in series with said third flow pass, further downcomer means connecting the upper rear furnace wall and convection enclo-3 sure panels in series with the roof.

Preferably buffer circuit panels are positioned between the front, rear and sidewall panels of the first and second passes, comprising a plurality of parallel vertically oriented tubes disposed in the comers of the furnace enclosure between the passes. Header means are provided to direct a sufficient portion of the flow from the inlet ends of both the first and second passes to the buffer circuit tubes so that the average temperature in the buffer circuit tubes is maintained intermediate the temperatures in the first and second passes.

In this way, the effects of thermal stress caused by restrained expansion differences in the all-welded enclosure between adjacent tubes of the first and second passes are minimized.

Accordingly, it is an object of the present invention to provide a generator circuit design in which the disadvantages of prior designs are overcome; and in particular generator design of simplified concept which is functionally equivalent or superior to past designs. By functionally equivalent, it is meant a circuit design which has a minimal fluid enthalpy pickup per pass and which by virtue of furnace pass geometry (including pass distribution and tube sizing) is relatively insensitive to flow imbalance caused by heat upset.

It is a further object of the present invention to provide a generator construction and design which is simpler and less expensive than those used heretofore, and in particular, which is less expensive to erect, requiring fewer field welds and minimum connecting piping and downcomers.

The invention, advantages thereof, and embodiments will become apparent upon further consideration of the following specification, with reference to the accompanying drawings, in which:

FIG. 1 is a section elevation view of a vapor generator in accordance with the present invention;

FIG. 2 is a schematic perspective view illustrating the furnace circuitry and construction of the generator of FIG. 1, in accordance with the present invention;

FIG. 3 is a partial schematic perspective'view further illustrating a furnace circuit and construction of the generator in accordance with an embodiment of the invention;

FIG. 4 is a flow diagram for the generator of FIGS. l-3 in accordance with the concepts of the present invention;

FIG. 5 is a perspective view of a portion of a tube wall section in accordance with the present invention;

FIG. 6 is a temperature-enthalpy diagram illustrating the operation of the generator of FIG. 1;

FIG. 7 is a schematic perspective view illustrating the furnace circuitry and construction of the generator in accordance with an embodiment of the present invention;

FIG. 8 is a flow diagram illustrating the arrangement of surfaces of the generator of FIG. 7; and

FIG. 9 is a temperature-enthalpy diagram illustrating operation of the generator of FIG. 7.

Turning to the FIGS. the vapor generator in accordance with the present invention is broadly indicated with the letter A and comprises a vertically extending rectangular-shaped radiant furnace portion B having an upper exit end C to which is connected a horizontally extending vestibule portion D and a downwardly extending convection area E; these last two areas comprising the convection section of the generator. Burners F are disposed in the furnace portion immediately above a bottom hopper G. The flow of hot gases is'upwardly in the furnace portion B to the vestibule portion and downwardly extending convection area of the generator, to the generator outlet H; and from there to a conventional air heater I for heat exchange between the hot gases and incoming air for the burners.

Turning to FIGS. 1-5 in particular, the vapor generator radiant furnace portion comprises an upright rectangular enclosure ]2 defined by front and rear walls 14 and 16 between which are disposed sidewalls l8 and 20 (FIG. 2), the enclosure extending vertically from the bottom hopper G to a roof 22 (FIG. 1 Immediately beneath the roof 22, the rear wall 16 is divided or branched to provide an exit screen 24 leading to the vestibule D of the generator, and a floor 26 leading to a second screen 28, these last two items with the exit screen 24 constituting an enclosure which encompasses tubes of a finish ing superheater, item 30, to be described. By bending a selected number of tubes from the rear wall for the enclosure floor 26 and screen 28, there is adequate spacing across the exit screen 24, as well as screen 28, for the flow of hot gases.

The vestibule D in addition to floor 26 comprises opposed sidewalls, not shown in FIGS. 1 and 2; whereas the downwardly extending convection area E comprises a partition wall 32, a front wall panel 34 extending downwardly from the floor 26, a rear wall panel 36, and sidewalls (also not shown). The partition wall 32 divides the convection area E into front and rear gas passes 38 and 40. As shown, the roof 22 extends not only above the furnace enclosure 12 but also above the vestibule D and convection area E.

Within the vestibule D and convection area E, encompassed by the heat recovery area tubes as defined above, are a primary superheater 42, in the rear gas pass 40; the finishing superheater 30 encompassed by the furnace second pass screen tubes 24 and 28; plus a bank of reheater tubes 44 in the enclosure front gas pass 38. Economizer tubes 46 are also disposed in the rear gas pass 40, beneath the primary superheater bank, the circuitry further including a radiant platen superheater 48 positioned in the furnace upper portion adjacent to and in front of the furnace exit screen 24.

It is a feature of the invention that a membrane-type wall construction illustrated in detail in FIG. '5 will be used substantially throughout in walls of the generator, formed by welding together a plurality of finned tubes 50 along their lengths so that the enclosures are substantially gastight. By virtue of the membrane-type wall construction, use of a conventional refractory and casing-type construction, with accompanying costs, is substantially avoided. 1

In accordance with the present invention, the circuitry is completed by providing in the upper portion of the furnace enclosure of the generator, a partial division wall 52 comprising a plurality of L-shaped division wall panels 54. Each panel comprises a plurality of parallel tubes defining a substantially horizontally extending leg 56 penetrating the rear wall 16 of the furnace enclosure immediately beneath the floor of the vestibule, and a substantially vertically extending leg 58 penetrating the roof 22 of the enclosure, the panels being arranged in spaced-apart vertical planes parallel with the generator sidewalls and between the front and rear walls. As shown in FIG. 2, the division wall panels may be six in number occupying a substantial area of the furnace upper end C above the plane of the burners F. The horizontal and vertical legs are connected, respectively, to headers 60 and 62, outside of the furnace enclosure, which, in turn, are connected in the circuitry, in a manner to be described, so that the inlet flow from the economizer 46 is first to the divisionwall panels and then to the circuits of the furnace enclosure.

By connecting the division wall panels in the circuitry so that they are upstream of the furnace circuits, and so that the flow from the economizer is first to the division wall panels, they are thus water cooled and capable of more heat pickup for a given surface area than a comparative surface cooled with a fluid at a higher enthalpy or temperature level.

Although the use of one or more full division wall panels, or division wall panels of other configuration, in series with and upstream of the furnace enclosure wall passes, is within the scope of this invention, the L-shaped construction for the division wall has many advantages, which will become apparent; but at this time, it is appropriate to give several advantages.

For one, a full division wall, which extends the full height of the furnace enclosure, divides the furnace into two vertically extending but separate cells, and if two full division walls are used, the furnace is divided into three vertically extending cells. This presents the problem of dividing the burners equally between the cells, but even if the cells have the same number of burners, it is difficult, particularly in the case of coal firing where a single pulverizing mill will usually be connected to several burners, to control -the mill outputs and burner firing rates so that the heat inputs into the cells are the same. The use of partial division wall panels in the upper part of the furnace clearly overcomes this problem.

In addition, compared to a full division wall, the multiple partial division wall panels provide a means for placing more surface area within the enclosure, permitting reduction in the size of the enclosure for a given absorption of heat; one object of the present invention. As an example, by use of the division wall panels, the overall saving in furnace height may be as much as 65 feet, for a generator of about 280 feetin height.

As a further advantage, the inlet headers 60, of the division wall panels, positioned close to the rear wall 16 of the fumace enclosure, above the plane of the burners, are close to the lo-' only a very short connection (item 64 FIG. 1) between the economizer and division wall inlet headers is required, as compared to the long very expensive downcomer which would be required if a full division wall were used.

Further in the case of very large furnaces, the tubes of a full division wall would have to be relatively larger and have a correspondingly thicker tube wall.

The fluid circuitry in accordance with the invention is illustrated in FIG. 4. From the economizer 46, the connection 64 (discussed above) conveys the flow directly to the inlet headers 60 of the division wall panels for parallel flow therein. From the outlet headers 62 of the division wall panels, downcomers 66 are provided to take the flow downwardly (FIGS. 1 and 2) to the lower inlet headers 68 for the furnace front and rear wall panels 14, 16, for parallel upward flow therein. As shown in FIG. 2, the headers 68 and panels l4, 16 are substantially coextensive with the furnace front and rear walls, these walls thereby constituting the furnace enclosure first flow pass. Referring back to FIG. 4, from outlet headers 70a and 70b for the front and rear walls, and also header 70c, FIG. 2, for the screen tubes 28, (it will be recalled that the rear wall 16 divides into exit screen tubes 24 and 28, terminating in the headers 70b, 70c), the flow is transmitted to suitable downcomers 72 and via the downcomers to lower inlet headers 74 for the sidewalls 18, 20. The flow is then parallel upwardly in the furnace sidewalls to upper outlet header 76 exiting to connections leading to the roof tubes 22.

The roof tubes 22 are integrally connected with the rear wall panel 36 of the convection area E, so that the flow in the roof tubes terminates in a header 78 at the bottom of the rear wall. At this point, suitable connections are provided leading to lower inlet headers 80 (FIGS. 1 and 4) for the heat recovery area convection enclosure, the flow then passing in succession to the primary, platen and finishing superheaters 42, 48, and 30.

With reference to FIG. I, it is apparent that the flow is upwardly in the furnace walls, in essentially two vertically oriented upflow flow passes connected in series, each pass comprising pairs of opposed walls. In this way, the panels of each pass are in similar absorption zones, the burners being disposed in the opposite front and rear wall panel 14 and 16. Each wall panel of pass one thereby will be subjected to roughly the same radiant heat input from the burner arrangement. Similarly the sidewall panels of pass two, because of their symmetrical location relative to the burners, will absorb roughly equal amounts of heat.

It is also an aspect of the invention that by positioning the burners in the front and rear walls, the burners are in and face the tubes of the colder first pass, providing added protection against tube overheating in the circuitry.

FIG. 6 is a temperature-enthalpy diagram which illustrates the temperature and enthalpy increase in the generator of FIG. 1 as the fluid progresses through the successive flow passes, at supercritical pressure of about 3,600 p.s.i. This is at full firing rate, the design or sizing of the generator furnace enclosure and partial division wall panels being that necessary to obtain at the furnace gas exit the maximum permissible flue gas temperature for the fuel fired. In that this is a coal fired unit, the gas exit temperature, depending upon the grade of the coal, is in the order of 2,350 F. Because of this design choice the enthalpy pickup in the economizer, is from about 550 B.t.u.s per pound to about 640 B.t.u.s per pound, increasing in the partial division wall panels to about 800 B.t.u.s per pound. In the first furnace pass, the enthalpy is further increased to about 980 B.t.u.s per pound, and in the second furnace pass, on up to about 1,140 B.t.u.s per pound.

Because of the substantial enthalpy pickup in the division wall panels, partly a result of the temperature difference between the water cooled panels and the hot gases in this part of the furnace, and partly a result of the added surface which can be provided with this type of division wall, the above enthalpy pickups in the furnace enclosure first and second passes are sufficiently low so that the passes exhibit stable flow 0[b for mixing with the flow from downcomers 66 and concharacteristics; that is, should a heat absorption upset occur, the enthalpy pickup in each of the enclosure passes is sufficiently low that a large flow imbalance is not likely to result in either of the passes.

The distinction of the present invention over that described in the copending application, Ser. No. 794,629, should now be apparent. Since the maximum permissible flue gas (furnace) exit temperature for coal firing is less than for oil or gas firing, the furnace enthalpy pickup must be greater. The application of a water cooled division wall to the furnace circuitry upstream of the first and second passes "to absorb a substantial portion of the heat released in the furnace maintains the enthalpy pickup in the first and second passes within the desired limits necessary for furnace circuit stability.

As a further aspect in accordance with the invention, since the enthalpy pickup required of the furnace enclosure passes is minimized, the passes are relatively small in dimension and utilizing standard tube diameters, at least in the lower high absorption portion of the furnace enclosure, mass flow rates in the tubes of the passes are correspondingly high, in the order of about 2X 10 to about 2.5 10 lb./hr.-sq. ft. Accordingly, by virtue of the pass geometry, and the above mass flow rates, the furnace enclosure circuitry in accordance with the invention is one which is relatively insensitive to flow imbalances should they occur; that is, the mass flow rates are sufficient to provide proper cooling of the tubes in the event of a small flow imbalance.

As a further advantage, limiting the furnace enclosure to two passes, in addition to permitting the passes to be geometrically spaced, results in a reduction in the number of downcomers and connections required, reducing field welding, and costs in constructing the generator.

In addition, because the maximum permissible furnace gas exit temperature is used, the log-mean temperature difference for heat transfer from the flue gases to the superheater and reheater convection sections is held at highest values to minimize the surface requirements for these expensive sections. By means of this and other factors (For instance, it is proposed to employ the most densely packed surface arrangement as possible in the heat recovery portion of the generator, and the maximum amount of economizer surface possible within design limits), the overall generator rating or average absorption per square foot of surface area is maximized.

FIGS. 3 and 4 illustrate a preferred embodiment of the invention. As shown, the corners of the furnace enclosure consist of tubes of a buffer circuit, comprising four corner panels, 82, 84, 86 and 88 (only two which are shown in FIG. 3) interposed between the front, rear and sidewall panels 14-20 of the first and second passes. Flow into the bufier circuit panels is accomplished by providing inlet headers 90 substantially coextensive with these corner panels, and transmitting a portion of the flow from the division wall downcomers 66 (leading to front and rear wall headers) via four connections 900 (FIG. 4) to the buffer circuit inlet headers 90. A portion of the flow transmitted to the inlet headers 74 for the sidewall panels, from downcomers 72, is also transmitted to the inlet headers 90 for the bufier circuit panels via four connection nections 90a.

These combined flows produce a fluid flow into the buffer circuit inlet headers which is at a temperature intermediate the inlet temperatures into the first and second pass panels. At the outlet ends of the buffer circuit panels, the flow is simply into the furnace sidewall headers 76, for mix with the flow from the sidewalls.

By suitably orificing the connections to the buffer circuit panels, the flow can be balanced between these panels and the However, the expansion stresses between the panels of passes one and two will be relatively small. FIG. 6 illustrates this advantage of the invention. By positioning the division wall panels upstream of the furnace enclosure passes, the enthalpy pickup of the furnace enclosure passes is shifted to the relatively horizontal portion of the temperature enthalpy curve, between about 800 and 1,140 B.t.u./lb. (at supercritical pressure), so that the increase in temperature in the enclosure passes is at a minimum, from about 710 F. to about 770 F. This is particularly the case during normal load operation of the generator, and only during startup or at low load, are the temperatures between adjacent tubes of the first and second passes sufficient to warrant use of the buffer circuit panels in the comers of the furnace enclosure.

Further details on the buffer circuit may be had with reference to the copending application Ser. No. 794,629, or reference to prior US. Pat. No. 3,344,777, applied for by Walter P. Gorzegno and assigned to assignee of the present application, describing the concepts of a buffer circuit in detail. It is sufficient to note at this time, that the improvement of the present invention, as in the copending application, is in positioning the buffer circuit panels in the critical corner portions of the generator.

As mentioned above, it is desirable to maintain high mass flow rates in the furnace pass tubes, to render the passes relatively insensitive to a flow upset, requiring the use of relatively small diameter tubes in the furnace enclosure; but at the same time, it is desirable to use as large diameter tubing as possible to reduce the pressure drop in the furnace portion of the generator. In the copending application Ser. No 794,629, there was described an improved arrangement in which the tube diameters were increased in the upper part of the furnace enclosure, in an area subjected to a lesser radiant heat input; so that in the lower high-absorption part of the enclosure, the mass flow rates are maintained at a high level whereas, in the upper part of the enclosure, the mass flow rates are reduced, to lower the overall pressure drop in the furnace circuitry. This concept of the copending application is to be incorporated by reference into the present application.

An embodiment in accordance with the invention is illustrated in FIGS. 7-9. As shown, the furnace enclosure first and second passes occupy only the lower part of the furnace, exiting to headers spaced from the bottom of the enclosure a distance equal to about two-thirds of the height of the enclosure, and a third flow pass is provided making up at least part of the upper part of the enclosure. In particular, the enclosure first pass panels 94 and 96 occupy the lower part of the front and rear walls of the enclosure, two-thirds their flow from the division wall panels 98 via downcomers 102 and inlet headers 104 and 106. From the outlet headers 108 and 110, the flow is returned to the bottom of the furnace via downcomers 112 and 114 for passage into the lower inlet headers 116 and 118 of the sidewall panels 120 and 122 of the furnace, the latter making up the second furnace enclosure pass. The headers 124 and 126 for these panels are positioned on the same plane as the headers 108 and 110 for the first pass panels, outside of the sidewalls about two-thirds of the height of the furnace from the bottom. A third flow pass 128 is U -shaped occupying the upper sidewalls and the upper front wall of the furnace enclosure, and the flow from the outlet headers 124 and 126 for the lower sidewall panels is distributed into the upper pass via a Ushaped header 130 for the pass, for upward flow in the pass to an upper U-shaped header 132 above the furnace enclosure. 1

An advantage of this arrangement, is that the third pass increases the flow area for the fluid after it has undergone a substantial enthalpy increase, reducing the velocity of the flow in the upper part of the furnace enclosure and the overall pressure drop in the generator. In other words, the use of a third pass in the upper part of the generator serves the same function as the enlarged tube diameter concept mentioned above, and described in copending application No. 794,629. The arr'angement is particularly important with the use of division wall panels upstream of the fumace enclosure circuitry which cause the flow in the upper part of the furnace enclosure to be in an advanced stage of heating.

FIG. 9 illustrates this aspect of the invention, the partial division wall being sized to increase the enthalpy pickup to about 800 B.t.u.s per pound. In the furnace first pass, the enthalpy pickup is only to about 905 B.t.u. '3 per pound, and in the second pass, to only about 1,005 B.t.u.'s per pound. At the outlet of the third furnace pass, the enthalpy input is increased to about 1,110 B.t.u.s per pound, about the same total furnace enthalpy pickup as for the two pass circuitry of FIG. 6. In both the first and second passes, the enthalpy increase is within the limits necessary to obtain stable circuit characteristics. i

FIGS. 7 and 8 also illustrate other aspects of this embodiment of the invention. From the upper furnace enclosure U- shaped outlet header 132, the flow is to the roof 134 (FIG. 8) of the generator, as with the embodiment of FIG. 1, the roof tubes extending across the furnace, vestibule and convection area and being bent also to make up the rear wall (not shown) of the convection enclosure exiting at a lower header at the bottom of the convection enclosure rear wall, as with the embodiment of FIG. 1. However, as distinguished from the embodiment of FIG. 1, the flow from the convection enclosure rear wall is then transmitted by means of suitable connections not only to the heat recovery convection enclosure side, front and division walls, but also to the upper: rear wall 136 of the furnace via an inlet header 138 therefor. From the rear upper furnace wall outlet headers 140 including the outlet header 142 for screen 144, and outlet headers for the convection enclosure side, front and division walls 146, the flow is than to the primary, platen and finishing superheating sections 148, and 152 (FIG. 8) as with the embodiment of FIG. 1.

As an alternate arrangement, the furnace screen tubes entering the upper furnace rear wall header, 140, may be made part of upper furnace pass 3, that is, bein parallel flow with the upper furnace front and sidewalls.

Among other advantages, the arrangements of both embodiments of the invention provide a much simplified furnace construction with reduced numbers of connections and downcomers, reducing the overall cost of the generator.

Although the invention has bee described with reference to particular embodiments, variations within the scope of the following claims will be apparent to those skilled in the art.

We claim:

1. A supercritical once-through vapor generator comprismg:

a radiantly heated rectangular furnace enclosure including a plurality of parallel vertically oriented tubes defining front, rear and sidewall panels; I

header means connecting said tubes to define first and second flow passes in series How and side-by-side relationship, said wall panels at least in the lower periphery of the furnace enclosure being divided into said first and second flow passes with each pass occupying opposite walls of the enclosure;

division wall means in said enclosure; and

connection means connecting said division wall means so that the supercritical fluid flow is first to said division wall means and then to the first and second flow passes.

2. A supercritical once-through vapor generator comprisa radiantly heated rectangular furnace enclosure including a plurality of parallel vertically oriented tubes defining front, rear and sidewall panels;

header means connecting said tubes to define first and second flow passes, the front and rear wall panels at least in the lower portion thereof constituting the first flow pass, the sidewall panels at least in the lower portion thereof constituting the second flow pass;

said enclosure comprising a lower burner zone, and an upper gas exit;

a convection heat transfer area in flow communication with the furnace enclosure gas exit;

water cooled division wall means including at least one panel of parallel tubes positioned in the furnace enclosure; and

connection means connecting said division wall means so that the supercritical fluid flow is first to the division wall means and then to the furnace enclosure first flow pass.

3. The generator of claim 2 wherein said furnace enclosure and division wall means are sized to limit the enthalpy pickup therein so that the maximum permissible flue gas exit temperature for the fuel fired obtained.

4. The generator of claim 3 including an enclosure roof above said gas exit wherein said division wall comprises a plurality of side-by-side panels positioned between the sidewall panels of the furnace enclosure, including lower substantially horizontally extending legs penetrating an enclosure wall, and upper approximately vertical upstanding legs penetrating the enclosure roof.

5. The generator of claim 4 including burner means positioned in said generator lower burner zone, said burner means being adapted for coal firing.

6. The generator of claim 2 including a third furnace enclosure flow pass additional header means connecting said generator third flow pass to receive the flow from the generator second flow pass, said third flow pass comprising panels of the generator enclosure upper sidewalls and front wall, thereby being U-shaped the furnace enclosure first and second flow passes comprising panels of the furnace enclosure lower front, rear and sidewalls.

7. The generator of claim 6 wherein said third flow pass occupies approximately the upper third of the furnace enclosure front and side wall panels, and the first and second flow passed occupy approximately the lower two-thirds of the furnace enclosure wall panels.

8. The generator of claim 3 further including platen superheating surface means positioned in said furnace enclosure adjacent the gas exit thereof, and finishing superheating surface means positioned in said convection heat transfer area.

9. The generator of claim 8 wherein said division wall panel is L-shaped said platen superheating surface being disposed approximately in the space which is embraced by the legs of said division wall panel.

10. A once-through vapor generator comprising:

a radiantly heated furnace enclosure including a plurality of parallel vertically oriented tubes, defining front, rear and side wall panels; I

header means connecting said panels in series into first and second flow passes, the first flow pass constituting front and rear wall panels and the second flow pass constituting sidewall panels;

a high absorption burner zone defining the bottom portion of said furnace enclosure and a lower absorption gas exit zone defining the upper portion of the furnace enclosure;

said gas exit zone including a gas exit;

a convection area in gas flow communication with the gas exit zone through said gas exit;

a plurality of partial division wall panels positioned in the furnace enclosure gas exit zone including lower approximately horizontally extending legs penetrating an enclosure wall and approximately vertically extending legs penetrating the enclosure roof;

header connection means connecting said division wall panels upstream of the furnace enclosure first flow pass;

said generator including platen, primary and finishing superheating means, the primary and finishing superheating means being positioned in the convection area of the generator, said platen superheating means being positioned in the furnace enclosure upper gas exit zone approximately in the space embraced by said division wall panels; and

said division wall panels and furnace enclosure being sized to limit the enthalpy pickup therein so that the maximum permissible flue gas exit temperature for the fuel fired is obtained at said gas exit. 

